1. Field of the Invention
The present invention relates to the field of Neighborhood Area Networks (NANs), and more specifically to NANs incorporating coaxial cable to increase the data rate and distance of the NAN.
2. Description of the Related Art
Local area networks (LANs), including ATM, Frame Relay, and Token Ring, are very successful in providing data communication and are used throughout the industry. Numerous LANs use the Ethernet standards set forth by the IEEE 802.3. Under this standard, 100Base-TX (100 Mbit) baseband data signals are transferred over unshielded twisted pairs (UTP) such as Cat-5. Under Ethernet standards, transmission distances are limited to the IEEE 802.3-specified 100 meter maximum. The 100 meter maximum is based upon CSMA/CD collision domain and Cat-5 attenuation criteria. When significantly longer distances are required, fiber optic cable is employed.
Fiber optic cable has much higher capital and outdoor installation costs in order to protect the delicate fibers. Fiber transceiver nodes are also more expensive than for UTP, such as Cat-5. Consequently, economics have encouraged the widespread proliferation of 100Base-TX Cat-5-based LAN systems. Fiber optic systems are typically used only for longer trunk lines between work groups. Fiber cable is also limited in that it is a dielectric material and cannot conduct electrical power. Thus, fiber cable cannot be used to supply electric power to remote sites.
LANs usually employ a conventional AC power system to supply power to hubs, repeaters, switches, and other network components. Installation of certain components must be made in locations that are not conveniently located near AC power sources. In such conditions, power must be supplied either by installing separate power wires, by using batteries, or by employing the Cat-5 wiring itself.
A Cat-5 cable contains four (4) twisted pairs with a first pair for uplink signals and a second pair for down link signals. The third and/or fourth pairs are usually unused but may be used for power distribution. The amount of power that can be so transferred is limited by the resistance of the Cat-5 wire, which may be approximately 9–10 ohms per 100 meters. The amount of transferable power is also limited by the dielectric and electrical code limitations on voltage range. The voltage range may be approximately 24–60V, depending on the jurisdiction.
In practicality, the total equivalent resistance for the power circuit comprises the sum of the source and return wire resistances. For example, a six (6) Watt switch operating at 3.3 V consumes nearly 2A, which in turn dictates that a 100 meter length of Cat-5 cable would drop nearly 34 V and 60 Watts in the wire alone. U.S. Pat. No. 5,994,998 describes an alternate technique for using the signal pairs in a Cat-5 cable to carry power. This reduces the resistance by a factor of two (2). However, this technique is impractical for longer distances because of the degradation of the signal to noise ratio. Such degradation is caused by increased noise generated by power supply current through greater lengths of cable in addition to the decreased signal level. Another disadvantage of this technique is the high cost of quality inductors. The relatively expensive inductors are required to isolate signals from power without seriously altering the delicate IEEE 802.32u specification for 100Base-TX signals.
Another data transmission system providing power distribution is the Community Antenna TV (CATV) systems. The CATV is not a LAN in the Ethernet sense and usually carries signals as modulated radio frequency (RF) in the down link direction rather than bi-directional baseband data signals. The CATV employs a power distribution system to power RF amplifiers at periodic distances to compensate for attenuation at VHF and UHF frequencies. Amplifier system power is usually supplied by AC or DC power distribution systems comprising either: 1) the outer shield and inner conductor of the cable itself; or 2) separate power wires running with the cable.
U.S. Pat. No. 3,987,240 describes a CATV power system of the first method in which DC power of one polarity is coupled to the center conductor of a coaxial cable. The opposite polarity is coupled to the shield of the coaxial cable. The high frequency TV signals are also conducted over the same coaxial cable with high quality chokes and filters used to separate the power from the RF signals at the amplifiers. The disadvantage of the first method is the relatively high cost and quality of the chokes and filters required to effectively separate the power from the RF in the face of power system noise and transients. The second method uses separate power distribution wires to avoid the costs of quality chokes and filters.
As networking of computers increases, the demand for NANs has likewise increased. A NAN differs from a LAN in that a NAN averages greater distances between users and requires a server having greater capacity. A NAN may therefore exceed the 100 meter maximum length specified by the IEEE for Cat-5 cable. A NAN further differs form a LAN in that the NAN requires that a power distribution system be integrated with the data distribution system to power boosters, repeaters, hubs and switches. A NAN also differs from a LAN in that the NAN deploys outdoor aerial and underground burial techniques and technology. Both aerial and underground burial techniques are required to give a NAN the flexibility for networking residences and businesses in neighborhoods.
Given the requirements of NANs, the conventional Cat-5 cable with RJ-45 connectors has too many inherent limitations to be acceptable for use in many NAN embodiments. As an alternative, coaxial cable has advantages over Cat-5 cable such as lower impedance and lower attenuation. For example, the attenuation of low loss coaxial cable is 60% less than that of Cat-5 (to 100BaseTX signals) and provides a 40–50% increase in range. Coaxial cable is also mechanically a hardier connection due to the thicker spacing in its shielding and sheath. Moreover, coaxial cable is more robust and more resistant to physical impact than Cat-5 cable. Coaxial cable is, therefore, superior in enduring environmental conditions, including without limitation underground burial. In light of the advantages of coaxial cable, the typical NAN would benefit from successful incorporation of coaxial cable therein. However, conventional specifications do not support the use of coaxial cable in a NAN.
It would, therefore, be an advancement in the art to provide a robust NAN system that extends data transmission distances beyond industry standards. It would be a further advancement in the art to provide a NAN system that uses coaxial cable to better resist environmental conditions. It would be yet another advancement in the art to provide a NAN system that interfaces with Cat-5 cable. Moreover, it would be an advancement in the art to provide such a system at a relatively low cost. Such a system is disclosed and claimed herein.